1. Field of the Invention
This invention relates to fluorescent compositions and low-velocity electron excited fluorescent display devices utilizing the same, and more particularly relates to fluorescent compositions comprising a conductive material having a specific particle diameter distribution and a specific phosphor in a specific mixing ratio, and low-velocity electron excited fluorescent display devices containing these fluorescent compositions as a fluorescent screen.
2. Description of the Prior Art
As is well known, a low-velocity electron excited fluorescent display device (hereinafter referred to as "fluorescent display device") in general has a fundamental structure such that both an anodic plate having a fluorescent screen on one side thereof and a cathode standing face to face with the above-described fluorescent screen are enclosed in an evacuated tube. The fluorescent screen placed on the anodic plate is excited by low-velocity electrons emitted from the cathode to result in emission of light. Both FIGS. 1 and 2 give outlines of typical structures of fluorescent display devices, and they show a diode type display tube and a triode type display tube, respectively. As shown in both FIGS. 1 and 2, one side of an anodic plate 11 made of, for example, an aluminium plate, has a fluorescent screen 12 thereon. The other side of the anodic plate 11 is supported by a ceramic base plate 13. The diode type display tube is equipped with a cathode standing face to face with the above-described fluorescent screen 12 placed on the one side of the anodic plate 11, and emission of light occurs by excitation of the fluorescent screen 12 which arises from low-velocity electrons emitted from the cathode 14. In particular, the triode type display tube shown in FIG. 2 additionally has a grid electrode 15 between the cathode 14 and the fluorescent screen 12 so as to control or diverge low-velocity electrons emitted from the cathode 14. Moreover, when the surface of the fluorescent screen 12 has wide area, two or more cathodes may be additionally placed in both fluorescent display tubes shown in FIG. 1 and FIG. 2 wherein only one cathode is placed, and there is no particular limit to the number of cathodes that can be placed therein. The aforesaid anodic plate 11 having a fluorescent screen 12 on one side thereof, the ceramic base plate 13 and the cathode 14 (which are shown in FIG. 1), or the aforesaid anodic plate 11 having a fluorescent screen 12 on one side thereof, the ceramic base plate 13, the cathode 14 and the grid electrode 15 (which are shown in FIG. 2) are enclosed in a transparent container 16 made of, for example, glass, the pressure inside which is held at a high vacuum of 10.sup.-5 to 10.sup.-9 Torr.
The following fluorescent compositions have heretofore been known as phosphors which can emit light of high luminance under low-velocity electron excitation;
a blue emitting fluorescent composition comprising indium oxide (In.sub.2 O.sub.3) and a silver activated zinc sulfide blue emitting phosphor (ZnS:Ag) in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Public Disclosure No. 22911/1977).
a blue emitting fluorescent composition comprising zinc oxide (ZnO) and ZnS:Ag in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Application Laid Open No. 115787/1977),
a green emitting fluorescent composition comprising In.sub.2 O.sub.3 and a green emitting phosphor selected from the following group in a mixing weight ratio ranging from 1:9 to 9:1, said group consisting of a copper and aluminium activated zinc cadmium sulfide phosphor [(Zn.sub.1-b,Cd.sub.b)S:Cu,Al, wherein b is 0.ltoreq.b.ltoreq.0.1], a cerium activated yttrium aluminium gallium oxide phosphor [Y.sub.3 (Al.sub.1-a,Ga.sub.a).sub.5 O.sub.12 :Ce, wherein a is 0.ltoreq.a.ltoreq.0.5], an europium activated strontium gallium sulfide phosphor (SrGa.sub.2 S.sub.4 :Eu.sup.2+), a manganese activated zinc silicate phosphor (Zn.sub.2 SiO.sub.4 :Mn) , a terbium activated yttrium oxysulfide phosphor (Y.sub.2 O.sub.2 S:Tb) and mixtures thereof (Japanese Patent Public Disclosure No. 239114/1977),
a green emitting fluorescent composition comprising In.sub.2 O.sub.3 and a terbium activated lanthanum yttrium oxysulfide green emitting phosphor [(La.sub.1-x,Y.sub.x).sub.2 O.sub.2 S:Tb, wherein x is 0.ltoreq.x.ltoreq.1] in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Public Disclosure No. 46916/1977),
a green emitting fluorescent composition comprising ZnO and SrGa.sub.2 S.sub.4 :Eu.sup.2+ in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Public Disclosure No. 46913/1977),
a green emitting fluorescent composition comprising ZnO and a green emitting phosphor selected from the group consisting of (Zn.sub.1-b,Cd.sub.b)S:Cu,Al, Y.sub.3 (Al.sub.1-a,Ga.sub.a).sub.5 O.sub.12 :Ce, Zn.sub.2 SiO.sub.4 :Mn, (La.sub.1-x,Y.sub.x).sub.2 O.sub.2 S:Tb and mixtures thereof in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Application Laid Open No. 104481/1972),
a red emitting fluorescent composition comprising In.sub.2 O.sub.3 and a red emitting phosphor selected from the following group in a mixing weight ratio ranging from 1:9 to 9:1, said group consisting of an europium activated yttrium oxysulfide phosphor (Y.sub.2 O.sub.2 S:Eu), an europium activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu), an europium activated yttrium vanadate phosphor (YVO.sub.4 :Eu) and mixtures thereof (Japanese Patent Public Disclosure No. 23916/1977), and
a red emitting fluorescent composition comprising ZnO and a red emitting phosphor selected from the group consisting of Y.sub.2 O.sub.2 S:Eu, Y.sub.2 O.sub.3 :Eu, YVO.sub.4 :Eu and mixtures thereof in a mixing weight ratio ranging from 1:9 to 9:1 (Japanese Patent Application Laid Open No. 145479/1976).
Although the above-mentioned blue, green and red emitting fluorescent compositions emit blue, green and red light of high luminance under low-velocity electron excitation occurring under acceleration potential below 1 KV, particularly below 100 V, further improvement of the luminance of the light emitted from these fluorescent compositions is presently desired from the viewpoint of practical use thereof. Further, phosphors which can emit yellow light of high luminance under low-velocity electron excitation occurring under acceleration potential below 1 KV, particularly below 100 V are demanded as the practical use of fluorescent display device increases. However, such yellow emitting phosphors have not heretofore been known.